Spinal implants such as interbody fusion devices are used to treat degenerative disc disease and other damages or defects in the spinal disc between adjacent vertebrae. The disc may be herniated or suffering from a variety of degenerative conditions, such that the anatomical function of the spinal disc is disrupted. Most prevalent surgical treatment for these conditions is to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for a portion of the annulus, by way of a discectomy procedure. A spinal fusion device is then introduced into the intradiscal space and suitable bone graft or bone substitute material is placed substantially in and/or adjacent the device in order to promote fusion between two adjacent vertebrae.
Certain of these spinal interbody fusion devices are of a fixed size and configuration with a hollow internal graft chamber and openings through superior and inferior surfaces for communicating with endplates of opposing vertebral bodies. Graft material to promote fusion is typically packed into the chamber so as to facilitate fusion with the adjoining vertebral endplates through the openings. Examples of such devices are described in U.S. Pat. No. 4,834,757, issued to Brantigan on May 30, 1989 and U.S. Pat. No. 5,702,449, issued to McKay on Dec. 30, 1997. The McKay '449 patent shows a fusion implant with an outer sleeve and an interior body of porous biocompatible material for permitting bony ingrowth therethrough.
Richelsoph in U.S. Pat. No. 5,749,916, issued on May 12, 1998 noted that unless the graft material inside the device is properly stressed, the quality of the fusion may be affected. Richelsoph recognized that according to Wolf's Law bone grows along lines of stress. In Richelsoph's view the prior fixed dimension devices simply hold the vertebral bodies apart and act as spacers with the material within the devices never being stressed. In order to produce stress on the graft material Richelsoph provided a device that is flexible and has transfer means for transferring stress from the adjoining vertebrae to the graft material within the device.
It has also been recognized that fusion can be enhanced between adjacent vertebrae and graft material by stretching the ligaments attached to adjoining vertebral bodies in a process called ligamentotaxis as described, for example in U.S. Pat. No. 6,709,438, issued to Dixon et al. on Mar. 23, 2004. With ligamentotaxis the implant may be maintained in position under compression applied by adjacent vertebrae due to the tension induced in the stretched ligaments. Such compression encourages fusion by the loading of the graft interface with adjacent vertebral body endplates. In such a procedure, the graft has been of fixed dimension and placed after the opposing vertebrae had been spread.
With certain known expandable interbody fusion devices, graft material is placed within the device or the intradiscal space prior to expansion. The graft material tends to lose any stress loading within the implant as the implant is expanded. As a result, graft material is introduced into the device during or subsequent to expansion and around the expanded device. Examples of such devices include those described in U.S. Pat. No. 8,062,375, issued on to Glenn et al. on Nov. 22, 2011, U.S. Pat. No. 8,105,382, issued to Olmos et al. on Jan. 31, 2012, U.S. Pat. No. 8,382,842, issued to Greenhalgh et al. on Feb. 26, 2013, and U.S. Pat. No. 8,403,990, issued to Dryer et al. on Mar. 26, 2013. In certain other instances bone graft is introduced under pressure in an effort to stress load the graft material against the endplates of opposing vertebral bodies, as described, for example in commonly assigned PCT Application WO 2013/036707 A1, published on Mar. 14, 2013.
Accordingly, it is desirable to provide an improved expandable spinal interbody fusion device that prior to insertion contains material for promoting bone fusion and that is capable of providing and maintaining stress loading on such material during and after expansion, without the placement of additional graft material into the device during or after expansion.